In U.S. patent application Ser. No. 707,796, U.S. Pat. No. 4,622,905, there is disclosed an improvement in the operation of a rotary hearth furnace which has an impermeable, horizontally rotating hearth heated from above by energy radiating from flames. The present invention is particularly pertinent to such a rotary hearth furnace in which any particular point on the hearth surface travels in a circular path from a first locus at which items to be heat processed are placed on the hearth to a second locus at which those items now heat processed are removed from the hearth and in which objects being heat processed are at rest relative to the hearth during travel from the first to the second locus. The production capacity of such a hearth furnace is governed by the linear speed of the hearth and the maximum number of layers of objects which can be placed on the hearth assuming radiant energy to be in excess. In the case of the furnace described in U.S. application Ser. No. 707,796 the objects to be heat processed are ball-like masses of metal oxide material in association with a carbonaceous reductant (e.g. Fe.sub.2 O.sub.3 +coke). In this particular instance, the ball-like objects perhaps 2 to 3 cm. in diameter can only be placed upon the hearth approximately three-deep and still achieve adequate heat processing (e.g. reduction of metal oxide) during normal hearth travel. If fourth, fifth etc. layers are loaded on the hearth at the first locus, only the top three layers will be adequately heat processed because all layers under the top three will be in the shadow of the top three layers and will remain relatively cool. While the observation of adequate heat processing of only three layers resting on an impermeable hearth is strictly applicable only to roughly spherical objects, those skilled in the art will appreciate that a similar consideration of shadowing will govern the maximum number of static layers of any particularly shaped objects heat processed by subjecting them to radiant energy.
In order to increase the productive capacity of the hearth as disclosed in U.S. application Ser. No. 707,796 one might consider adding mechanical lifting means to the hearth to periodically displace the upper and lower layers of objects being heat processed or converting the furnace to a permeable hearth structure involving passage of hot gas through the hearth and pellet bed to heat by forced convection-conduction rather than by radiation. Both of these options are very expensive and somewhat unreliable when operating a furnace at temperatures above 1000.degree. C. containing a corrosive atmosphere, e.g. water vapor, CO and CO.sub.2. Furthermore, as to the first option, if, during heat processing, the whole or the surface of objects being processed softens, it is likely that adhesion of objects could occur which would obviate the effect of the lifting means.